Hydrophilic SAMs

Figure 4c shows that the amount of adsorbed proteins is rapidly saturated within several minutes of exposing serum-containing medium to a surface. Albumin, the most abundant serum protein, was expected to preferentially adsorb onto the surfaces during early time points. Then, adsorbed albumin was expected to be displaced by cell adhesion proteins. To investigate the effect of preadsorbed albumin displacement on cell adhesion, SAMs were first exposed to albumin then, HUVECs suspended in a serum-supplemented medium were added [21, 42]. Very few cells adhered to hydrophobic SAMs that had been pretreated with albumin, due to the large interfacial tension between water and the hydrophobic surfactant-like surface. Albumin was infrequently displaced by the cell adhesive proteins Fn and Vn. One the other hand, HUVECs adhered well to hydrophilic SAM surfaces that had been preadsorbed with albumin. In that case, the preadsorbed albumin was readily displaced by cell adhesive proteins. 

Figure 6.1 illustrates such an example (Abbott et al. 1992). Here, a gold surface is first coated with a hydrophilic SAM of alkanethiolate. The desired geometric pattern is then formed on the surface through micromachining (in this case, using a surgical scalpel), and the resulting features are covered with a hydrophobic SAM (of dialkyl disulfide) (see Fig. 6.1a). This procedure can be used to construct micrometer-scale hydrophobic lines on the surface so that the resulting shapes and distribution of liquid drops can be controlled (see Fig. 6.1b).

Figure 11A shows an optical micrograph of water drops preferentially condensed on hydrophilic SAMs terminated by carboxylic (COOH) groups no water condensed on the hydrophobic SAMs terminated by methyl (CH3) groups [98]. This process shows how the functionality of a SAM influences the eondensation of water vapor on a SAM-derivatized surface. It uses self-assembly at two scales the formation of SAMs at the molecular scale and the directed condensation of water vapor at the macroscopic scale. The organization of liquids into patterned arrays illustrates one of the uses of self-assembly in microfabrication [99]. 

Fig. 3.5 Fabrication of pp specimens for SPM calibration. The first step (1) comprises soft-lithography of appropriate SAM molecules onto a planar substrate. A composite stamp, which has both flat and corrugated areas, allows printing of the comb-patterned strip with the adjacent solid calibration field. Next, a graded UV-ozonolysis (UVO) systematically modifies the chemistry of the patterned SAM (and calibration field) along one direction (3). For example, methyl-terminated alkyl chain SAMs (hydrophobic) can be gradually converted into carboxylic acid terminated (hydrophilic) chains. Subsequent filling with a hydrophilic SAM completes the matrix of the specimen (2fA...

Cox JD, Curry MS, Skirboll SK, Gourley PL, Sasaki DY (2002) Surface passivation of a microiluidic device to glial cell adhesion a comparison of hydro-phobic and hydrophilic SAM coatings. Biomaterials 23 929-935... 

It is also possible to produce more hydrophilic SAMs using aqueous solutions ofthe ammonium salt of hydroxy-terminated dodecyl phosphate. Furthermore, the wettability (water contact angle) imposed onto a metal oxide siuface can be varied in a controlled, predictable way by the formation of mixed methyl-Thydroxy-terminated dodecyl phosphate SAMs, self-assembled from aqueous solutions containing both molecules in an appropriate ratio. Investigations of the relationship between adsorbate composition and solution amphiphile concentration ratio for mixed SAM systems (by XPS and ToF-SIMS) as well as amphiphile orientation and order in the different SAMs (by NEXAFS and AFM) are currently under way. 

Figure 6.11. Illustration of assembly of high-surface-tension fluids on a carboxylic-acid-terminated SAM. The stamp shown in Figure 6.10 was used to produce 10 pm diameter circles of a hydrophilic SAM on to which a UV-curable polymer was assembled...

Figure 1.15 High-resolution lateral surface characterization and the associated water structure for a hydrophilic SAM in pure water, (a] 12 x 12 nm area of the SAM showing faint periodicity and striped regions. The region indicated in the top-right corner corresponds to the location of the water structure measurement shown in [b], [b] Dissipation as a function of sample displacement away from the tip [zero on the x-axis is arbitrary). Oscillations in the dissipation signal are separated by approximately 2.2 A and are associated with the displacement of individual water layers between the tip and the sample [reused with permission from Jarvis copyright...

Fig. 17. Surface Potential Acj) and surfece tension change Ay as a function of the bulk salt concentration for various sodium halides at the [(a) (c)] hydrophobic and [(b) (d)] hydrophilic SAM-water interface.

Coupling of affinity molecules to surfaces also can be enhanced by the use of discrete PEG linkers. Nishimura et al. (2005) modified an amino surface with a NHS-PEG -maleimide crosslinker to create a hydrophilic self-assembled monolayer (SAM) surface that was thiol reactive for the conjugation of sulfhydryl-modified RNAs. This array then was used to investigate the binding specificity of synthetic kanamycins with selected RNA sequences to prove the specific interaction of ribosomal RNA with this molecule. The PEG linkers on surfaces provide lower nonspecific binding character than alkyl linkers, when preparing SAM surfaces for affinity interactions. 

Recently, Melosh has obtained electrically stable LAJs as large as 9 mm2 by atomic deposition of a nanometer-thick passivating layer of aluminium oxide on top of self-assembled organic monolayers with hydrophilic terminal groups [158,159]. Obviously, interlayers based junctions limit electrical measurements only to organic SAMs less conductive than the protecting layer. 

Fig. 5.1 Scheme for patterned deposition of PbS. (a) Self-assembled monolayer (SAM) of long-chain mercapto-carboxylic acid. On exposure to UV radiation through a mask, the exposed thiol group is oxidized to a sulphonate group (b) that is weakly bound to the An substrate and can be easily rinsed away (c). Subsequent formation of a long-chain alkyl thiol SAM occurs only on the exposed Au (d). CD of PbS occurs only on hydrophilic car-boxylate endgroups and not on hydrophobic methyl groups (e). (See Ref. 41). 

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